Roadway planers, also known as pavement profilers, road milling machines or cold planers, are machines designed for scarifying, removing, mixing or reclaiming, material from the surface of bituminous or concrete roadways and similar surfaces. These machines typically have a plurality of tracks or wheels which support and horizontally transport the machine along the surface of the road to be planed, and have a rotatable planing cylinder that is vertically adjustable with respect to the road surface.
On cold planers that integrate the machine chassis with the planing cylinder, as described in U.S. Pat. No. 4,186,968, issued Feb. 5, 1980, to Robert M. Barton and currently assigned to the assignee of the present invention, the entire chassis is raised or lowered to control the depth of cut of the cutting bits into the ground surface. If the cutting bits strike a high density inclusion, such as a manhole cover or railroad track during the planing operation, an event known as a "kickback" can occur.
A kickback event sensor that senses fluid pressure in a hydraulic circuit regulating the height of an adjustable strut member on the cold planer is described in U.S. Pat. No. 4,929,121 issued May 29, 1990 to Kevin C. Lent et al. and assigned to the assignee of the present invention. The control system described in this reference employs a signal produced by a pressure switch, in response to a kickback event, to sequentially disengage the cutter, or planing cylinder, from the drive engine.
When a kickback event occurs, the planing cylinder on a typical down-cutting machine will attempt to rise up out of the cut. In a similar manner, changes in material density can cause the chassis on an up-cutting machine to also rise up out of the cut. If the cold planer is operating with an automatic grade control system, such as the portable string line system described in U.S. Pat. No. 4,270,801 issued Jun. 2, 1981 to George M. Swisher, Jr. et al, the automatic grade control, sensing that the machine is above the desired grade, will attempt to lower the chassis by retracting the supporting strut members, leaving the machine principally supported on the rotor. In this position, the machine cannot be steered or braked because of insufficient contact between the strut mounted tracks, or wheels, and the ground. In this condition, the operator may not be able to stop, steer, or control undesirable movement of the machine.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a kickback event control arrangement that, in the event of a kickback, will maintain sufficient vehicle weight on the ground support members to permit the operator to maintain operational control of the movement of the machine. It is also desirable to have a method of controlling the operation of a cold planer so that sufficient weight is maintained on the ground support members to inhibit loss of operational control of the machine during the occurrence of a kickback event.